The present invention relates to a contour correction circuit for displaying a sharp picture free from degrading picture quality regardless of any kinds of scenes supplied to a video display apparatus including a television receiver, and also relates to a contour correction method.
The technique to realize sharper pictures has been pursued since the advent of television receivers (TV) in the market. Recently, the screen of TV has become larger and larger, which emphasizes particularly an importance of this technique, i.e. the technique of enhancing sharpness of a reproduced-image on the screen. Further, digital technique introduced in TV allows accurate time control as well as fine control over picture quality, so that further sharper and clearer pictures are demanded.
A conventional contour correction circuit is described hereinafter with reference to the accompanying drawings.
FIG. 7 is a block diagram of a conventional contour correction circuit. FIG. 8 illustrates an operational concept of the circuit shown in FIG. 7.
In FIG. 7, corrected-waveform-generating-circuit 1 generates contour correction waveform 205 by differentiating video-input-signal 200 in twice. Variable amplifier 2 receives an output signal from generator 1 and controls amplitude of contour correction signal 215 responsive to a size of control signal 210 determined by a picture quality controller. Picture quality controller 3 determines a gain of amplifier 2. Delay circuit 4 synchronizes contour correction signal 215 tapped off from amplifier 2 with video input signal 200. Adding circuit 5 adds delayed video input signal 220 tapped off from delay circuit 4 to contour correction signal 215 so that video output signal 225 with sharpness is supplied. The operation of the circuit discussed above is described hereinafter.
Corrected-waveform-generating-circuit 1 receives input video signal 200. As shown in FIG. 8, signal 200 is twice differentiated in generator 1 so that contour corrected waveform 205 is generated. Variable amplifier 2 amplifies corrected waveform 205 responsive to a size of control signal 210 determined by picture quality controller 3. Contour corrected signal 215 is thus supplied from amplifier 2. Delay circuit 4 delays the input video signal to be synchronized with signal 215 delayed by generator 1 and amplifier 2. Adding circuit 5 adds delayed video input signal 220 tapped off from delay circuit 4 to contour correction signal 215 so that video output signal 225 with sharpness due to contour correction is supplied.
However, in the circuit structure discussed above, once the picture quality controller determines a control voltage at a certain level, amount of contour correction stays constant regardless of any input video signals. Therefore, in the case of a blank picture, e.g. when a picture shows only a blue sky, noises are conspicuous.
In order to address this problem, a method for automatically controlling the contour correction amount with regard to a detected amount was proposed. The detected amount corresponds to a fine portion out of an entire picture, e.g. lawn portion in the entire picture, the lawn portion having a little change of the amplitude corresponding to the edge portion of the input video signal.
The Japanese Letters of Patent No. 2532605 teaches the method as discussed above in FIG. 5 and FIG. 6 illustrating a conventional contour correction circuit.
Input video signal 200 is fed in parallel into delay circuit 4, first corrected waveform generating circuit 11, second corrected waveform generating circuit 12 and high pass filter 15. Delay circuit 4 adjusts the timing of input video signal 200 to meet a delay of the circuit system, and outputs the resultant delayed video signal 200 to adding circuit 10.
First-corrected-waveform-generating-circuit 11 produces contour-emphasis-signal 230 with regard to a signal having a high contrast ratio, i.e. a large change in amplitude corresponding to a crisp edge portion of signal 200, and supplies signal 230 to first variable amplifier 13. A noise component included in input video signal 200 is too little to be carried by signal 230. Amplifier 13 controls the amplitude of signal 230 responsive to a voltage determined by controller 3, and outputs the resultant signal to adding circuit 10 as the amplified-contour-emphasis-signal 240.
Second-corrected-waveform-generating-circuit 12 produces fine-contour-emphasis-signal 235 with regard to a small amplitude portion out of signal 200, and sends it to second variable amplifier 14.
Amplifier 14 controls the amplitude of emphasis signal 235 responsive to the sum of output voltages from controller 3 and smoothing circuit 21. Adding circuit 10 adds signal 240 and signal 245 to signal 220, then outputs video signal 225 undergone the contour correction process.
A method of obtaining fine-portion-controlling-voltage 280 that controls a gain of second variable amplifier 14 is now described hereinafter.
First, high pass filter 15 extracts high-frequency-portion out of input video signal 200 so that high-band-video signal 250 is formed.
Second, full-wave-rectifying circuit 16 rectifies high band video signal 250 so that rectified high-band-video-signal 255 is formed. First clipping circuit 17 removes subtle signals from signal 255 so that clipped and rectified high-band-video-signal 260 is obtained.
Then, signal 260 is fed into limiting circuit 18 and second clipping circuit 19 respectively. Limiting circuit extracts fine signals only so that fine-portion-video-signal 265 is formed.
Second clipping circuit 19 extracts edge portions having high contrasts so that contour video signal 270 is obtained. Fine-portion-video-signal 265 includes contour-portion besides fine-portion in a strict sense. Therefore, subtracting circuit 20 subtracts signal 270 from signal 265 to obtain an accurate fine-portion-signal-component, and then outputs fine-controlling-pulse-signal 275.
Smoothing circuit 21 smoothes signal 275 so that a voltage responsive to an amount of fine portions can be obtained. Accordingly, this voltage is lowered at a picture having relatively smaller fine-portion. Since this voltage controls the amplitude of the fine-portion-emphasis-signal 235, the fine-portion is not so much emphasized, and noises are reduced. On the contrary, when a picture has relatively greater fine-portion signals, the fine-portion is strongly emphasized so that the picture can be sharply enhanced.
The circuit structure discussed above still has the following problem.
Fine-portion-controlling voltage 280 averaged by the smoothing circuit controls the gain of second variable amplifier 14. Therefore, when a signal having a high contrast ratio such as input video signal 200 shown in FIG. 6 is adjacent to a signal having a low contrast ratio such as a signal corresponding to vague edge portion where an amplitude changes a little, fine-portion-controlling voltage 280 is corrected only at its average value. Thus the fine-portion-emphasis-signal is also added to output video signal 225 having a high contrast ratio as shown in FIG. 6.
As a result, a picture port ion having a high contrast is provided with unnecessary edge enhancement, and the noises of this portion are also emphasized.
The present invention addresses the problem discussed above and aims to provide a contour correction circuit having the following structure and also provide a contour correction method using the same circuit.
The contour correction circuit of the present invention comprises the following element:
(a) a delay circuit for delaying an input video signal;
(b) corrected waveform generating circuit for generating a contour correction signal using the input video signal;
(c) a variable amplifier for controlling an amplitude of the contour correction signal;
(d) contour control circuit for supplying a gain control signal that controls a gain of the variable amplifier during a time covering the waveform corresponding to edge portion of contour;
wherein the contour correction circuit includes;
(d-1) a scene determination circuit for removing noises and detecting an amplitude change as well as waveform width in an edge portion of the input video signal; and
(d-2) a control signal generator corrects the smaller amount of contour at the greater change in the amplitude when an output signal is over a given level, and
the control signal generator corrects the larger amount of contour at the greater change in the amplitude when an output signal is less than the given level;
(e) a first adding circuit for adding an output signal from the delay circuit and an output signal from the variable amplifier.
In other words, items (d-1) and (d-2) realize the following contour correction.
1. Detect a time corresponding to edge portion of a contour-picture-signal of an object in a picture as well as an amplitude change in the contour-picture-signal.
2. While the contour is drawn, correction amount thereof is controlled so that the smaller amount is corrected at the greater amplitude of the picture-signal.
As a result, users can always watch sharp pictures free from degradation of picture quality on TVs or other video display apparatuses regardless of whatever video scenes are fed thereto.
Further, a contour can be corrected flexibly to users"" tastes.